G2iii: (Mock) Describe the cardiovascular events that occur during ventricular systole
Systole = the period of the cardiac cycle where blood is ejected from the ventricles
- Events of systole divided into:
- IONIC
- MECHANICAL
- ECG ∆
Ionic
- Cardiac AP from SA Node depolarises myocyte
→ Threshold potential -65mV reached
→ Fast Na+ channels open = RAPID DEPOLARISATION
→ TRANSIENT OUTWARD K+ channels opens = INITIAL REPOLARISATION
→ L TYPE Ca2+ CHANNELS open @ –40mV = PLATEAU PHASE
Excitation-Contraction-Coupling
- This influx Ca2+ causes further Ca2+ release by myocyte SR
- ↑intracellular Ca2+
- Ca2+ binds TnC
- Conformational ∆ in Tropomyosin uncovers Actin Binding Site
- Allows myosin head to form a cross bridge
- Sarcomere shortens until ↓Ca2+ or ↓ATP supply
- Contraction of cardiac myocyte
ECG
- ISOVOL CONTRACTION
- spreads from AV node → Purkinje fibres
- Ventricular depol. = QRS complex
- Simultaneously atria are repolarising (atrial T wave not seen on ECG because covered by QRS)
- RAPID EJECTION
- Ventricles completely depolarised at beginning of ejection
- ISOELECTRIC ST SEGMENT on ECG
- REDUCED EJECTION
- Ventricular repolarisation → T wave
Mechanical Events
There are 3 phases:
- ISOVOL CONTRACTION
- RAPID EJECTION
- REDUCED EJECTION
1) Isovolumetric Contract
- All valves closed
- Ventricles depolarise
- Myocyte contraction causes ↑intraventricular P
- Ventricular P > atrial P = AV valves shut
- Rapid ↑ ventricular P without ∆ volume
- Maximal rate of ↑P KA dP/dt
2) Rapid Ejection
- Ventricular P > aorta & pulmonary pressures
- Ejection of blood
- Maximal systolic aorta & pulmonary pressures reached
3) Reduced Ejection
- Kinetic E of blood continues to propel blood outward, but at lower pressures